The present invention relates generally to multidimensional chromatography, and in particular to an on-line system and method which couples liquid chromatography to supercritical fluid chromatography in a way that includes large injection volume fluid handling for the supercritical fluid chromatography. This system and method are also capable of introducing large volumes to the supercritical fluid chromatography column by means other than liquid chromatography such as a syringe, an autosampler or a valve containing a sample loop.
The analysis of complex matrices, such as polymer additives, usually requires manual clean-up steps to remove undesirable components prior to analysis. In this regard, multidimensional chromatography offers the potential of rapidly accomplishing the necessary sample preparation or clean-up steps without manual intervention. In multidimensional chromatography, two or more separation techniques work in tandem, such that an initial separation step is followed by a further separation technique on a particular section or fraction of interest produced by the prior separation step. For example, one liquid chromatograph ("LC") could be coupled to another LC to provide multidimensional separation. Similarly, a liquid chromatograph ("LC") could be coupled to a gas chromatograph ("GC") to provide another form of multidimensional separation. However, LC-LC multidimensional chromatography can be limited in terms of sensitivity and by solvent incompatibilities. Additionally, while LC-GC multidimensional chromatography can be used successfully for analytes transportable in the gaseous mobile phase, there are quite a number of compounds which are not sufficiently volatile to be analyzed by GC.
In contrast, supercritical fluid chromatography ("SFC") enables compounds to be analyzed which are not sufficiently volatile to otherwise be analyzed by GC, such as materials having relatively high molecular weights. SFC also has the capability of utilizing a flame ionization detector ("FID") for permitting sensitive and universal detection. Thus, the creation of an LC-SFC multidimensional chromatographic system offers the potential of providing highly sensitive analysis of non-volatile compounds. However, in SFC, capillary columns (e.g., 50 .mu.m i.d.) are typically required to obtain adequate performance. As a result of the need to use such small diameter columns for separation, the injection volumes must also be very small. In this regard, the injection volumes are generally limited to a range from less than one nanoliters to just a few nanoliters. This limitation represents a substantial impediment to the development of an effective "on-line" LC-SFC coupled system, because relatively large solvent volumes are typically used in LC to provide the necessary separation. In this context, the term "on-line" refers to a multidimensional system which provides fluid communication between the system components so that the analytical process may be conducted on a continuous basis without resort to manual collection steps.
Sample injection volume limitations can also hinder SFC analyses where the sample is not eluted through a liquid chromatography system. In many situations SFC alone can provide adequate analytical data. If the analytes are contained in a sample with a large amount of solvent, however, then the analyst can either inject a large amount of sample in which case the solvent will flood the column, or inject the proper amount of sample in which case the quantity of analytes injected may be too small to be accurately detected. Neither alternative is acceptable.
Accordingly, it is a principal objective of the present invention to provide an on-line LC-SFC multidimensional chromatographic system and method which permits large solvent volumes to be employed for purposes of LC separation, without requiring the actual introduction of large fluid volumes into the supercritical fluid chromatograph.
It is another objective of the present invention to provide an on-line LC-SFC multidimensional chromatographic system and method which first removes the LC solvent and then traps the analyte before the analyte is introduced into the supercritical fluid chromatograph.
Still another objective of the present invention is to provide a sample introduction system which allows large volumes of dilute samples to be analyzed via supercritical fluid chromatography.
It is further objective of the present invention to provide an interface for the system which is capable of trapping the analyte in a confined region in order to minimize band broadening of the chromatographic peaks in the subsequent SFC step.